Creating Mesh - Vertices out of boundries

Hello everyone o/

I'm to make a program that initializes an Icosphere (A sphere that consists out of triangles that have the same size and distance) from the script, where I can set the number of iterations for the sphere. The first creation of the sphere with 20 triangles works fine. But if I try to add more triangles recursively, I get this error:

Failed setting triangles. Some indices are referencing out of bounds vertices. IndexCount: 240, VertexCount: 42

I'm confused about that: My vertices and my indices range from 0-41 and from 0-239. But there should be no further Vertex at the next position. Has anybody an idea, what I could have done wrong? What do I not see? :/

Here is my code:

 public class GenerateIcosphere : MonoBehaviour {
 
     public List<Vector3> newVertices;
     public List<Vector2> newUV;
     public List<int> newTriangles;
     public Material wireframe;
     public int recrusionLevel = 0;
 
     private int triLevel = 0;
     private Dictionary<Int64, int> middlePointIndexCache;
     private Mesh iSphere;
 
     private void createIcosphere()
     {
         middlePointIndexCache = new Dictionary<long, int>();
 
         float t = (float)(1.0 + Mathf.Sqrt(5.0f) / 2.0f);
 
         newVertices = new List<Vector3> { new Vector3(-1, t, 0), new Vector3(1, t, 0),
                                        new Vector3(-1, -t, 0),new Vector3(1, -t, 0),
 
                                        new Vector3(0, -1, t), new Vector3(0, 1, t),
                                        new Vector3(0,-1, -t), new Vector3(0, 1, -t),
 
                                        new Vector3(t, 0, -1), new Vector3(t, 0, 1),
                                        new Vector3(-t,0, -1), new Vector3(-t, 0, 1)};
 
 
         newTriangles = new List<int> {0, 11, 5, 0, 5, 1, 0, 1, 7, 0, 7, 10, 0, 10, 11,
                                   1, 5, 9, 5, 11, 4, 11, 10, 2, 10, 7, 6, 7, 1, 8,
                                   3, 9, 4, 3, 4, 2, 3, 2, 6, 3, 6, 8, 3, 8, 9,
                                   4, 9, 5, 2, 4, 11, 6, 2, 10, 8, 6, 7, 9, 8, 1};
     }
 
     bool checkForDifference(int n)
     {
         if(n > triLevel || n < triLevel)
         {
             triLevel = n;
             return true;
         }
 
         return false;
     }
 
     void iterateIcosphere()
     {
         
         for (int i = 0; i < recrusionLevel; i++)
         {
             List<int> addTriangles = new List<int> { };
             for (int j = 0; j < newTriangles.Count; j = j+3)
             {
                 int a = getMiddlePoint(newTriangles[j], newTriangles[j+1]);
                 int b = getMiddlePoint(newTriangles[j+1], newTriangles[j+2]);
                 int c = getMiddlePoint(newTriangles[j+2], newTriangles[j]);
 
                 addTriangles.Add(newTriangles[j]);
                 addTriangles.Add(a);
                 addTriangles.Add(c);
 
                 addTriangles.Add(newTriangles[j+1]);
                 addTriangles.Add(b);
                 addTriangles.Add(a);
 
                 addTriangles.Add(newTriangles[j+2]);
                 addTriangles.Add(c);
                 addTriangles.Add(b);
 
                 addTriangles.Add(a);
                 addTriangles.Add(b);
                 addTriangles.Add(c);
                 
             }
 
 
             newTriangles = addTriangles;
         }
     }
 
     private int addVertex(Vector3 p)
     {
         float length = Mathf.Sqrt(p.x * p.x + p.y * p.y + p.z * p.z);
         newVertices.Add(new Vector3(p.x / length, p.y / length, p.z / length));
         return newVertices.Count;
     }
 
     private int getMiddlePoint(int p1, int p2)
     {
         //check if the point already exists
 
         bool firstIsSmaller = p1 < p2;
         Int64 smallerIndex = firstIsSmaller ? p1 : p2;
         Int64 greaterIndex = firstIsSmaller ? p2 : p1;
         Int64 key = (smallerIndex << 32) + greaterIndex;
 
         int ret;
         if (middlePointIndexCache.TryGetValue(key, out ret))
         {
             return ret;
         }
 
 
 
         //if not calculate new point
         Vector3 point1 = newVertices[p1];
         Vector3 point2 = newVertices[p2];
         Vector3 middle = new Vector3(((point1.x + point2.x) /2.0f), 
                                      ((point1.y + point2.y) / 2.0f), 
                                      ((point1.z + point2.z) / 2.0f));
 
         int i = addVertex(middle);
 
         middlePointIndexCache.Add(key, i);
 
         return i;
     }
 
     void Start () {
         createIcosphere();
         iSphere = new Mesh();
         gameObject.AddComponent<MeshFilter>().mesh = iSphere;
         iSphere.vertices = newVertices.ToArray();
         iSphere.uv = newUV.ToArray();
         iSphere.triangles = newTriangles.ToArray();
         iSphere.RecalculateNormals();
         gameObject.AddComponent<MeshRenderer>();
         gameObject.GetComponent<MeshRenderer>().material = wireframe;
     }
 
     // Update is called once per frame
     void Update () {
         if (checkForDifference(recrusionLevel))
         {
             iterateIcosphere();
             iterateUVs();
             iSphere = new Mesh();
             gameObject.GetComponent<MeshFilter>().mesh = iSphere;
             iSphere.Clear();
             iSphere.vertices = newVertices.ToArray();
             iSphere.uv = newUV.ToArray();
             iSphere.triangles = newTriangles.ToArray();
             iSphere.RecalculateNormals();
             
         }   
     }